1. Field of Invention
The present invention relates to cellular communication; and more particularly to a method and system for effecting handoff to one of a plurality of neighboring cells.
2. Description of Related Art
A cellular communication system includes several base transceiver stations, each of which transmits and receives channels of RF information throughout a predetermined coverage area referred to as a cell. The outer boundary of a cell is determined not only by the effective radiated power of the base transceiver, which determines the maximum range at which a mobile unit is able to receive; but also, by the effective radiated power of the mobile telephones, which determines the maximum range at which the base station is able to receive. The base transceiver stations (BTS) determine their corresponding cell's handoff boundaries by setting an RF signal threshold value which must be met or exceeded by a mobile unit entering the cell in order to transfer an on-going call to that cell.
In many cellular installations, each BTS or mobile unit has several scanning receivers that periodically, or on command, measure the RF signal strength of selected traffic channels in order to determine the range of active mobile telephones relative to the transceiver of the cell in which the mobile telephone is located, referred to herein as a serving cell, and the proximity of the telephone to neighboring cells, which overlap the serving cell.
Based upon the received RF signal strength of the mobile unit by the transceiver of the serving cell, the BTS controls the effective radiated output power (ERP) of the mobile unit so that the transmission by the mobile unit does not saturate the base station transceiver when it is close to the center of the cell, but is of sufficient power to enable the BTS to receive the mobile transmission when it is far from the transceiver. When the RF received signal strength of a traffic channel of a neighboring cell measured by the scanning receiver in the serving cell exceeds the set RF signal threshold value that is configured into the system; then the system switches on-going communication to a traffic channel of a neighboring cell, thus, effecting a handoff.
Typically, when the RF signal strength of the traffic channel being used by the mobile unit decreases below a certain level and the RF signal strength of a neighboring cell measured at the mobile unit or the serving cell, increases above a certain level, the system specifies this particular neighboring cell for handoff. Because of the nature of cellular communications each cell has a number of neighboring cells, any one of which could be used to handoff a cellular communication. Thus, under certain conditions, a mobile unit is unable to handoff to the designated neighboring cell, which then requires the designation of another cell for the unit. This of course at times involves the queuing of mobile units awaiting their turn in accessing a particular cell. At certain times, the process of initiating a handoff is time consuming, which results in queuing delays and system congestion. Also, these delays cause the mobile units, under crowded conditions, to be subjected to critical signal strengths before handoff may be effected. In order to overcome these delays, a substantial amount of additional equipment is required which increases the cost of the system.
In light of the foregoing, there is a need for a method and system of handoff in a cellular communication system which overcomes the drawbacks and shortcomings hereinbefore mentioned.